Prior art chairs having a seat bucket consisting of a seat part and back part (DE-GM 75 00 152) with a recess in the seat bucket are known. The recess in these constructions includes a laterally extending slot arranged approximately parallel to the front edge of the seat and lying in the back part at the approximate region of the transition to the seat part. Two additional slots running approximately perpendicular from this slot toward the front of the seat part are provided so that the recess has approximately the shape of a U. Owing to this, a tilting of the back part about a horizontal axis extending through the front end points of the slots running in the sitting direction is effected. However, since the main laterally extending slot forming the base of the U extends over the whole width of the seat, the slot concerned must be covered by an elastic overlay if bruising of the buttocks of the person sitting on the chair is to be prevented. Such a slot design can therefore not be used for a bucket structure without additional padding or the like.
In known swivel chairs (DE-GM 84 01 000) the seat, in the region of its front as well as of its rear end, is capable of tilting about axes aligned parallel to the front and back seat edges. In these constructions the back of the chair is formed separately from the seat and is capable of tilting in itself in such fashion that the so-called angular opening of the seat, that is, the angle between the seat and the back is variable within relatively wide limits when the person sitting on the chair changes his position, for example, when moving from a straight sitting posture to a position leaning far backward. Ergonomic studies have shown that this is particularly favorable for counteracting symptoms of fatigue in that the spinal column is effectively supported by the back of the chair in all sitting positions. Such a chair, however, has a relatively complicated and costly structure. For example, the seat and back must be connected separately on the chair frame. Also, a plurality of axes of rotation, in some cases with slot guidance and spring return elements, is necessary in the region of the front and back ends of the seat and because of these features a correspondingly complicated and costly chair frame becomes necessary.
Also known are so-called free-swinging chairs, that is, those in which the frame, seat and back are formed of continuous springy tubular sections which with a simple structure do indeed permit a relatively good adaptation to various sitting postures, but yet at the same time are disadvantageous in so far as when a person occupying the chair leans far backward. When this happens, the region of the front edge of the seat is displaced backward and downward and thereupon likewise the position of the person with respect to surrounding pieces of furniture, particularly table tops. In addition, variation of the angular opening of the seat is limited and a bucket-shaped design of chair and back is not possible.